Neurodegenerative diseases such as, for example, Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis (ALS) are characterized by the death and/or degeneration of neurons and neural tissue. Because terminally differentiated neurons are limited in their ability to proliferate, a possible method of treatment for conditions characterized by neural degeneration is to provide new neural cells to replace those lost by cell death or injury. To this end, neural cell transplantation has been attempted in animal experiments using embryonic or adult neural stem cells, ES cells and embryonic neural cells. However, such uses face major hurdles against their application in humans. The use of embryonic stem cells or neural cells is beset with ethical issues, and the question of guaranteeing a stable supply is also a concern. Although the demonstrated ability of ES cells to differentiate is currently attracting much attention, the cost and labor required to induce differentiation to specific cell types, and the risk of forming teratoid tumors after transplantation, are factors impeding stable application of this technology. On the other hand, patients undergoing transplantation of adult neural stem cells are exposed to a tremendous risk and burden, since these cells are found in a very limited core section of the central nervous system (i.e., deep in the brain) and thus must be extracted by craniotomy. Finally, attempts to treat neural degeneration and nerve injury by transplantation of other types of pluripotent cells, such as hematopoietic stem cells, have met with limited success, due to the difficulty of controlling the differentiation of the transplanted cells.
As a result of the aforementioned difficulties associated with the use of embryonic cells, adult neural stem cells and various adult pluripotent cells for the treatment of neurodegenerative diseases, new methods and compositions for treatment of neurodegenerative diseases are required.